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Saturday, 19 July 2014

From the earliest days of the manufacture of coal gas for lighting there has been an accompanying scientific discourse and experimentation on how the distillation of coal, and its products, could be used. The vast amount of work done by scientists in the early period of coal gas manufacture has barely been recorded - yet without it the industry could not have been so successful and, indeed, may not have taken off at all.

The paper below may seem to have little to do with coal gas manufacture - but it is part of an ongoing discourse on a wide range of products and applications by scientists from many disciplines and backgrounds.

For local Greenwich people - John McCulloch was a doctor and geologist. He was appointed as an assistant surgeon rising to Surgeon in the Royal Artillery and in 1803, became Chemist to the Board of Ordnance (which means he was in Woolwich - one of a very large number of distinguished scientists there). He also lived in Blackheath where he practised as a physician.

Geological Society meeting May 1st 1812

A paper by Dr. Mculloch (Member of the Society) On Bistre
and other substances produced in the distillation of wood; and on their analogy
with the native Bitumens, was read.

When wood is submitted to destructive distillation, there
is obtained, among other products, a black substance resembling common tar.
This tar is very inflammable, and so liquid that it may be burnt in a lamp. By
washing it with water either hot or cold, or submitting it to the action, of
lime or of the mild alkalis, a large portion of acetic acid is separated, and
the residue becomes pitchy and tenacious. It is entirely soluble in caustic
alkali, in alcohol, in ether, in acetic acid, and in the mineral acids. The fat
oils and the recent essential oils dissolve but little of it; but if the former
are made drying, and if the latter have become brown by keeping, they then act
more readily and copiously. Coloured oil of turpentine takes up a considerable quantity,
but naphtha only acquires a scarcely sensible brown colour by digestion upon
it. When carefully distilled at a gentle heat it is decomposed into an oily matter,
at first limpid and afterwards brown, a quantity of acetic acid combined with-a
little ammonia; and a spongy coal remains in the retort. In this process no
inflammable gas is given out; but at a high temperature the oil is more or less
decomposed, and an inflammable gas is producedwhich, however, does not burn with a flame by any means so bright as the
gas from pit-coal. If this destructive
distillation is not carried very far, the matter in the retortwill be found, when cold, to be-solid,
brilliant, shining, and possessed of a conchoidal fracture: its taste is burning
and pungent, and its' odour is that of wood smoke itis
fusible and readily inflammable. When kept melted in an open vessel till it
ceases to be fusible, it becomes more and more brilliant, its fracture passes
to splintery, and it assumes the perfect appearance of asphaltum. In proportion
as it approaches this state it becomes less and less soluble in alcohol, and at
length scarcely gives a stain to this menstruum. Naphtha has no action on it, and in this circumstance
alone it differs from asphaltum. Dr. M then
proceeds to an examination of the Bitumens and shows that the difference
between the products of recent vegetable matter, and of the bitumens when
subjected to distillation, consists in the former yielding empyreumatic acetic acid
and a black pitchy matter insoluble in naphtha; while the latter afford ammonia
and naphtha, but little or no acid. He then enters into a detailed
investigation of the proper lies of the very important class of Lignites. Of
those substances, such as peat, surturbrand, Bovey coal, &c; in which the
traces of vegetable origin are not obliterated.

Bovey brown coal gave an. oil resembling in odour, that
of wood-tar, but much more soluble in naphtha: that portion of the oil which was
insoluble in this menstruum had a strong odour of wood smoke. '

'The oil of jet was almost perfectly soluble in naphtha,
and smelled, strongly of petroleum; but it afforded also empyreumatie acetic
acid.

Thus it appears that there exists a class of fossils of
undoubted vegetable origin, which exhibit the gradual progress from wood to
bitumen, and in which this change has been brought about by the action not of
heat but of water.

The experiments, however, of Sir James Hall seem to show
that heat with compression is also capable of converting wood into coal.A critical examination of this fact was the
next object of Dr. M; and he found on heating wood in close gun-barrels that a
black coaly-looking sub stance was indeed produced, but that it consisted
wholly of charcoal, empyreumatic acid; and wood- tar, and did not contain the smallest
portion of real bitumen. Hence the experiments alluded to do by no means prove
the possibility of converting vegetable matter into real coal by mere heat. It
appears however to Dr. M that the consolidation of bituminous vegetable into coal
is not unlikely to be the effect of subterranean heat.

The paper concludes by showing the identity of the pitch
procured from the distillation of wood and the pigment called Bistre; and points
out methods of obtaining it in a state better fitted than common bistre for the
purposes of the artist; and also enumerates several other uses to which this
substance may be economically applied.

Thursday, 17 July 2014

The sketch on this page represents what is known as
the Tar Tower. The rectangular box-like vessel is the hydraulic main, which
acts as a collecting chamber for the gas from the separate retorts in the
setting.

The gas passes from the retort to the hydraulic main
by way of the ascension, bridge and dip pipes, three of which are shown.

The dip pipe projects downwards inside the hydraulic
main, the lower end being immersed m ammoniacal liquor to the extent of about a
quarter of an inch. This is known, as a "hydraulic seal," and its
purpose is to prevent gas from flowing from the hydraulic mains back through the
dip pipes to the retorts.

The gas is very hot when it leaves the retort, and carries
with it tar and water vapour, much of which condenses in the relatively cool
hydraulic main, and this condensate, together with other liquor which is added to
ensure a sufficiency, provides the" hydraulic seal" for the lower end
of the dip pipes.

The depth of the seal in each dip pipe must be the same
to ensure equal pressure conditions in all retorts in the setting. Provision is
therefore made for the removal of the (words missing)

The tower receives the condensate from a number of
hydraulic mains by way of a connection known as the tar main, and the liquor,
by reason of its higher gravity, sinks to .the bottom of the tower.

Inside the tower a cup or weir is fixed at the level
it is required to maintain in the hydraulic mains, and the separated liquor flows
through this cup and a scaled pipe to the storage tank. The tar is removed
periodically by way of the pipe at the bottom of the tower and flows into the
funnel and away to the storage well, liquor being supplied to maintain the level
in the tower.

A small pipe, known as the “equilibrium pipe"
connects the top of the towel to the “foul main" to maintain equal pressure
conditions in the tower and the "foul main" into which the gas flows
from the hydraulic main.